How to test underground feeder from PV array?

[jjhoward]

I have a run of 350' of 350MCM AL; 240V @ 90 amps supplied by 2 inverters. Each inverter supplying 40 amps max. Voltage drop calcs shows 3.81V along this 350' run. There are additional smaller portions of the circuit from the inverters to the utility meter. When I add in the other portions of the circuit the total Vd calculatin is 6.18V. The inverters are logging voltages that indicate a 10V or more Vd and of course the inverters shut down when they see a voltage that is too high. Any suggestions on how to confirm that the underground conductors are OK? Customer claims that the system was OK a year ago, but the inverters are regularly shutting down on sunny days. The inverters capture their power and voltage data for tracking system permformance. High voltage is seen and the inverters shut down. If a lighting strike damaged the aluminum feeders, how can I check them? Small resistance measurments are difficult...
Thank you.

 

[ggunn]

I have a run of 350' of 350MCM AL; 240V @ 90 amps supplied by 2 inverters. Each inverter supplying 40 amps max. Voltage drop calcs shows 3.81V along this 350' run. There are additional smaller portions of the circuit from the inverters to the utility meter. When I add in the other portions of the circuit the total Vd calculatin is 6.18V. The inverters are logging voltages that indicate a 10V or more Vd and of course the inverters shut down when they see a voltage that is too high. Any suggestions on how to confirm that the underground conductors are OK? Customer claims that the system was OK a year ago, but the inverters are regularly shutting down on sunny days. The inverters capture their power and voltage data for tracking system permformance. High voltage is seen and the inverters shut down. If a lighting strike damaged the aluminum feeders, how can I check them? Small resistance measurments are difficult...
Thank you.

Disconnect your inverters and megger your conductors; it doesn't seem likely to me that lightning could have damaged your conductors in such a way that it would increase their resistance but not compromise their insulation.

It may be that it is the service voltage which has changed rather than the PV system. Check your service voltage; if it is at the high end of the POCO's spec, your voltage rise may be enough to shut the inverters down. Check to see if the voltage window on the inverters is set as wide as it can be. In rare instances we have had to install buck-boost transformers between inverters and services to bring the voltage down into the inverters' operating window.

 

[jjhoward]

Yes, I guess I need a megger!
Even if the POCO has the service level at the high end (252v). The inverters are going up to 264. The known wire sizes & lengths don't show 10Vs or more. Solaredge app note says get that Vd to 1%, the calculated Vd of 618 is over the 1%, but where does 10 or more volts come from?? Yes, we have the inverters opened up, but they are still shutting down.

 

[jjhoward]

Buck boost xformer at the utility end of the run?

 

[jjhoward]

Sorry 618 v was meant to be 6.18v on previous post.

 

[jaggedben]

Buck boost xformer at the utility end of the run?

The utility might consider this rather unscrupulous since it will raise the voltage at the utility. That, if the utility voltage is too high, that problem should be brought to the utility's attention and addressed by the utility, not the customer. If the utility thinks the voltage is fine, and doesn't object to the buck boost as an alternative to larger conductors, then it could be okay.

 

[rlundsrud]

What about all your terminations? Was an antioxidant compound like Noalox used at the terminations? Have the torque specs been checked on all terminations? If you have this excessive voltage drop beyond your calculated values I think that's where I would start checking.

 

[ggunn]

What about all your terminations? Was an antioxidant compound like Noalox used at the terminations? Have the torque specs been checked on all terminations? If you have this excessive voltage drop beyond your calculated values I think that's where I would start checking.

If there is a significant difference, look for evidence of heating at the terminals. All that I squared R has gotta go someplace.

 

[jjhoward]

If there is a significant difference, look for evidence of heating at the terminals. All that I squared R has gotta go someplace.

There are numerous connections: AC out of inverters land on DP breakers in a small panel (connections 1 & 2, the inverter and the breaker).

6 awg CU lines from the main lugs of the panel (connection 3) into a trough where there are butt splices (hex lugs) connecting the 6 awg CU lines to 350 MCM Al. (connection 4)

At the end of the 350 MCM AL (350' away) there is another trough where the 350MCM AL is butt spliced onto 4 awg CU. (connection 5).

This is at a pool house, not the main house. The 4 awg CU runs ~10' to a fusable disconnect with time delay, 250 AMP fuses (not sure why). (connections 6 & 7, top & bottom of the disconnect).

From the disconnect 4 awg CU runs about 20' to a jbox where it is landed in a multitap block connecting a 3/0 CU (connection 8) with a 120' run to the main house and the POCO meter.
(Connection 9).
YIKES

 

[jaggedben]

You've got 90A on 6 awg cu? That's a code violation as well as a contributor to voltage drop. The 4awg cu might even be a violation too.

 

[gar]

180316-0730 EDT

jjhoward:

I don't think it should be too difficult to troubleshoot your voltage drop.

Test equipment needed ---
1. Two 120 V 1500 W portable space heaters. Would like equal current at sane voltage. Or use some 240 heating element. Goal is to get around a 10 A or higher load at 240 V.
2. Fluke 27 or equivalent. 87 if you want finer resolution. RMS or average reading, not a big concern.
3. Hall type or standard current transformer clamp-on current probe.
4. A long test lead, somewhat over your 350 ft. This could be #30 wire, but there would be no reason to go this small. Probably #16 or #14 so you can reuse it. I don't know if excess coiled on the spool will be a problem. Depends upon stray magnetic fields. The error from the measuring 1 turn loop of the conductor being tested and your stretched out long probe wire is probably not important.
5. Some way to directly probe conductors.
6. Possibly an extra person or so.
7. Some radio communication.


First test is to determine source impedance looking back at the power company.

Apply a 10 A or greater load change on the power company 240 V lines. Measure the load change (current), great accuracy is not required. You can probably live with 10%, but 1% would be better. Put your voltmeter probes directly on the conductors that come from the power company. Possibly this will be in the main panel. Contact the wires directly. Don't just measure on the terminating lugs. This is a 240 V measurement. This should be on the power company side of wherever the PV system connects.

Z utility source = delta V / delta I. Z utility source is everything from the point where you make the voltage change measurement back to power company generators, but mostly starts at the pole transformer. I would hope this Z is less than about 0.02 ohm, but it may not be. 100 A and 0.02 ohm is 2 V. Short circuit current about 12,000 A.


Second you connect your long wire probe directly to the wire from one side of the PV inverter, and to your voltage testing meter. The meter and its other probe will be taken to various test points. Note: this is the measurement of a small voltage drop along your conductor(s), not a change in supply voltage.

These tests to various points are best done by shutting down the PV system and using your load resistor at the PV end. Any major problem point should be obvious, otherwise you have a distributed problem.

You are only measuring 1/2 of the loop impedance this way, and thus both sides of the circuit must be looked at.

The load resistor method is easier to work with because there are no big variations such as may come from a varying PV system.

After you find and correct your problems, then remove the test load resistor, and do any required tests with the PV system as the source.

Note that breakers have substantial voltage drop.

.

 

[jjhoward]

180316-0730 EDT

jjhoward:

I don't think it should be too difficult to troubleshoot your voltage drop.

Test equipment needed ---
1. Two 120 V 1500 W portable space heaters. Would like equal current at sane voltage. Or use some 240 heating element. Goal is to get around a 10 A or higher load at 240 V.
2. Fluke 27 or equivalent. 87 if you want finer resolution. RMS or average reading, not a big concern.
3. Hall type or standard current transformer clamp-on current probe.
4. A long test lead, somewhat over your 350 ft. This could be #30 wire, but there would be no reason to go this small. Probably #16 or #14 so you can reuse it. I don't know if excess coiled on the spool will be a problem. Depends upon stray magnetic fields. The error from the measuring 1 turn loop of the conductor being tested and your stretched out long probe wire is probably not important.
5. Some way to directly probe conductors.
6. Possibly an extra person or so.
7. Some radio communication.


First test is to determine source impedance looking back at the power company.

Apply a 10 A or greater load change on the power company 240 V lines. Measure the load change (current), great accuracy is not required. You can probably live with 10%, but 1% would be better. Put your voltmeter probes directly on the conductors that come from the power company. Possibly this will be in the main panel. Contact the wires directly. Don't just measure on the terminating lugs. This is a 240 V measurement. This should be on the power company side of wherever the PV system connects.

Z utility source = delta V / delta I. Z utility source is everything from the point where you make the voltage change measurement back to power company generators, but mostly starts at the pole transformer. I would hope this Z is less than about 0.02 ohm, but it may not be. 100 A and 0.02 ohm is 2 V. Short circuit current about 12,000 A.


Second you connect your long wire probe directly to the wire from one side of the PV inverter, and to your voltage testing meter. The meter and its other probe will be taken to various test points. Note: this is the measurement of a small voltage drop along your conductor(s), not a change in supply voltage.

These tests to various points are best done by shutting down the PV system and using your load resistor at the PV end. Any major problem point should be obvious, otherwise you have a distributed problem.

You are only measuring 1/2 of the loop impedance this way, and thus both sides of the circuit must be looked at.

The load resistor method is easier to work with because there are no big variations such as may come from a varying PV system.

After you find and correct your problems, then remove the test load resistor, and do any required tests with the PV system as the source.

Note that breakers have substantial voltage drop.

.

Great response Gar, exactly what I was hoping to get.. Thank you. I will let you know how it works out!

 

[jjhoward]

You've got 90A on 6 awg cu? That's a code violation as well as a contributor to voltage drop. The 4awg cu might even be a violation too.

You are correct, it is 4 awg CU from the small panel to the trough then to 350 MCM AL.
At the pool house trough, it is 2 awg CU to the 3/0 CU for the 120' run to the main house.
Sorry..

 

[ggunn]

You are correct, it is 4 awg CU from the small panel to the trough then to 350 MCM AL.
At the pool house trough, it is 2 awg CU to the 3/0 CU for the 120' run to the main house.
Sorry..

Is 90A your total nameplate inverter output current? Even #4 is only good for 68A in continuous use if you have 75 degree terminals, which you probably do.

 

[ggunn]

Is 90A your total nameplate inverter output current? Even #4 is only good for 68A in continuous use if you have 75 degree terminals, which you probably do.

Even if your wire sizing is correct, you have a lot of connections, any one of which if loosened could add in some Vd.

 

[pv_n00b]

If the system was working then look at things that could change, the POCO voltage, conductor connection impedance, OCPD impedance, inverter running at a higher current, etc. The probability that the impedance of an undamaged conductor has changed along a run is pretty small. That would be the last thing I made an effort to check.

SolarEdge has an unreasonably low %VD requirement in their manual. I think when they came up with that they were thinking about residential installs where the inverter was next to the service panel. In larger systems, it's almost never going to be 1%. Going over 2% is problematic IMO. POCOs have been known to go higher than the ANSI standards. Double check, but you will need recording meters because the voltage might be spiking when another load on the line is switched.

 

[Ingenieur]

the 350' has a calculated drop of ~3 v
and you say a short section adds another ~3?
what conductor and length from inv to meters?

do this when the system is operating at a high output
measure v at inv and at meter, measure i at either
do the v at the same time, 2 guys, 2 meters
Z = (v inv - v metr)/i
compare it to Z values from tables

disconnect and measure the util v, maybe it's high?

 

[jjhoward]

Great response Gar, exactly what I was hoping to get.. Thank you. I will let you know how it works out!

Hello Gar;
I ran tests per you suggestions: had a 240V 30amp heater providing a constant load down at the PV site (at the combiner panel).
Measured voltages all along our collection of connections.
The fused disconnect up at the main house >400' from the inverters had a loose connection on one leg, producing a Vd of ~1.5 volt across the connection. We opened up the contact points, cleaned them up, No-ox and put it back together tighltly. Vd less than .1V
Also found that the street voltage was consistantly 254-255!. JCPL lowered the voltage to 240! All is good now.
Thank you Gar for the suggestion, thank you all for the review.
Cheers;
Joe

 

[JFletcher]

Thank you for returning to post the problem you found and the solution.

 

[ggunn]

Hello Gar;
I ran tests per you suggestions: had a 240V 30amp heater providing a constant load down at the PV site (at the combiner panel).
Measured voltages all along our collection of connections.
The fused disconnect up at the main house >400' from the inverters had a loose connection on one leg, producing a Vd of ~1.5 volt across the connection. We opened up the contact points, cleaned them up, No-ox and put it back together tighltly. Vd less than .1V
Also found that the street voltage was consistantly 254-255!. JCPL lowered the voltage to 240! All is good now.
Thank you Gar for the suggestion, thank you all for the review.
Cheers;
Joe

See my post #15.